What is called general-purpose polystyrene-based resin (GPPS), such as styrene homopolymer, excellent in tensile strength, heat resistance, light resistance, formability and surface glossiness, and high-impact polystyrene (HIPS) which is formed of GPPS with rubber, such as SBR and BR, added thereto to improve the brittleness of GPPS, have hitherto been in heavy usage as materials for beverage containers, yogurt containers, portion containers and food containers for pot noodle and the like. As a lid material to be stuck to the opening of such a polystyrene-based resin container to seal the opening, there has been used an aluminum laminate which includes aluminum foil as the base material and a sealant layer and the like provided on the surface of the base material for adhesion with a container. The opening of a container has hitherto been sealed with a small piece of aluminum lid material, which is blanked out of such an aluminum laminate into a spread form of a lid, with its end portion folded back into a skirt like shape. Such a lid made of aluminum has been regularly used because it is excellent in sealing properties and stability at the time of pealing, and has good sheet-feed properties with little adhesion by static electricity when fed to the opening of a container. Furthermore, once a skirt portion is provided by bending a circumferential edge portion, such a lid made of aluminum is excellent in what is called shape retention, i.e., the property that the bent and deformed shape is retained. For this reason, when a filled beverage is to be consumed directly from a container, it is possible to prevent the contamination of the area in the vicinity of the opening of the container because the portion in the vicinity of the opening of the container which comes into contact with the mouth is better maintained in a condition covered with an end portion of the lid. Thus a lid made of aluminum, which is excellent sanitarily and also excellent in terms of appearance, has hitherto been used in a preferable manner.
These days, however, there are often cases where foreign substances accidentally get into processed food containers, leaving challenges for safety measures. For a container with such a lid made of aluminum, since a metal detector cannot be used to check the entire product, there is a problem of inability to detect metals which might get into containers, such as nails, staples, broken pieces of molded article extraction blades, bolts and nuts, wires, and springs. Besides, for these containers with a lid made of aluminum, lids made of aluminum and container bodies made of synthetic resin, such as polystyrene, must be segregated and recovered, and hence these containers have the drawback of poor recyclability.
As a substitute for such a conventional lid made of aluminum, there has been proposed, for example, a lid material which is blanked out of a laminate material into a prescribed shape, the laminate material including a sealant material provided on the lower surface of a laminate base material, which includes a heat-resistant film laminated on opposite sides of a base material of a coextruded film which is provided with a center layer formed from high-density polyethylene and a polypropylene-based polymer and a coating layer formed from high-density polyethylene on opposite sides of this center layer (refer to patent document 1). Furthermore, the present applicants have proposed a sealed container having shape retention, which is obtained by cold molding polystyrene-based resin sheet which is such that the propagation energy in a 150-μm thick sheet measured by the weight-drop impact test method in accordance with ASTM-D3763 is not less than 0.015 J (refer to patent documents 2 to 6).
On the other hand, there have been proposed methods for heat sealing a container lid by using a container having a protruding heat-seal line on the upper surface of a flange of a container body or a container having a linear groove at the center of the upper surface of a flange of a container body (refer to patent documents 7 and 8). However, in the container described in patent document 7, the interior and exterior of the container are aerated to prevent complete sealing when the contents are substances that ferment, and in the container described in patent document 8, air is introduced into a groove not in communication with the exterior of the container, whereby poor sealing is prevented and an aluminum lid is caused to peel off readily.